Frequently, as in the case of automotive engine control, it is desirable to monitor the rotation of a shaft to measure parameters such as instantaneous shaft speed and absolute shaft position. Considerable speed variation can occur during each engine revolution and speed measurements on a tooth by tooth basis are important. It is preferred to utilize an existing ring gear having teeth used for meshing with another gear driven by a starter motor but usable as well for an angle encoder.
It is well known to measure the position of a shaft by sensing the teeth of a toothed wheel driven by the shaft to identify incremental shaft movement and to provide a reference mark corresponding to a shaft index position. A magnetic pickup adjacent the path of the teeth generates a generally sinusoidal electrical pulse for each passing tooth. Often the pulses are converted to square wave pulses for digital circuit analysis or pulse counting. To establish a reference or index position on the toothed wheel a separate tooth or notch and pickup are sometimes provided. Another strategy for developing a reference mark is to alter or omit a tooth to cause an extra pulse or a missed pulse which identifies the index position. Each of these latter arrangements spoils the pulse train for use as a speed sensor since a missed pulse or extra pulse would cause an aberration in speed measurement.
FIG. 1 shows a prior art system identified in Baumann U.S. Pat. No. 4,528,471 wherein a missing tooth causes a wide swing in the amplitude of an inductive pickup signal. When a zero crossing detector is used to develop a square wave several teeth go undetected so that the train of pulses are severely interrupted and a wide square wave pulse of unpredictable duration tu is produced, so that the position of the missing tooth is only imprecisely located The solution to that problem offered in the Baumann patent is shown in FIG. 2 There, a portion of each tooth top for two or three adjacent teeth is carved out so that each tooth maintains its mechanical usefulness but it appears to the pickup that one tooth is missing. In this case the signal is much less distorted than in FIG. 1, and one square wave pulse is skipped to yield a wide pulse t.sub.b. The reference point is still imprecise and the lost pulse causes a speed measurement error.